37 research outputs found
Tunnelling-induced cosmic bounce in the presence of anisotropy
If we imagine rewinding the universe to early times, the scale factor shrinks
and the existence of a finite spatial volume may play a role in quantum
tunnelling effects in a closed universe. It has recently been shown that such
finite volume effects dynamically generate an effective equation of state that
could support a cosmological bounce. In this work we extend the analysis to the
case in which a (homogeneous) anisotropy is present, and identify a criteria
for a successful bounce in terms of the size of the closed universe and the
properties of the quantum field.Comment: 8 pages, 5 figure
GRChombo : Numerical Relativity with Adaptive Mesh Refinement
In this work, we introduce GRChombo: a new numerical relativity code which
incorporates full adaptive mesh refinement (AMR) using block structured
Berger-Rigoutsos grid generation. The code supports non-trivial
"many-boxes-in-many-boxes" mesh hierarchies and massive parallelism through the
Message Passing Interface (MPI). GRChombo evolves the Einstein equation using
the standard BSSN formalism, with an option to turn on CCZ4 constraint damping
if required. The AMR capability permits the study of a range of new physics
which has previously been computationally infeasible in a full 3+1 setting,
whilst also significantly simplifying the process of setting up the mesh for
these problems. We show that GRChombo can stably and accurately evolve standard
spacetimes such as binary black hole mergers and scalar collapses into black
holes, demonstrate the performance characteristics of our code, and discuss
various physics problems which stand to benefit from the AMR technique.Comment: 48 pages, 24 figure
Oscillon formation during inflationary preheating with general relativity
We study the non-perturbative evolution of inflationary fluctuations during
preheating using fully non-linear general-relativistic field-theory
simulations. We choose a single-field inflationary model that is consistent
with observational constraints and start the simulations at the end of
inflation with fluctuations both in the field and its conjugate momentum.
Gravity enhances the growth of density perturbations, which then collapse and
virialize, forming long-lived stable oscillon-like stars that reach
compactnesses . We find that
increases for larger field models, until it peaks due to the
interplay between the overdensity growth and Hubble expansion rates. Whilst
gravitational effects can play an important role in the formation of compact
oscillons during preheating, the objects are unlikely to collapse into
primordial black holes without an additional enhancement of the initial
inflationary fluctuations.Comment: 7 pages. 4 figures. Movie: https://youtu.be/vTl9agMfPB0. Matches
version published in PR
A CCZ4 formulation for Einstein-Gauss-Bonnet gravity and ST in spacetime dimensions
We develop a modified CCZ4 formulation of the Einstein equations in
spacetime dimensions for general relativity plus a Gauss-Bonnet term, as well
as for the most general parity-invariant scalar-tensor theory of gravity up to
four derivatives. We demonstrate well-posedness for both theories and provide
full expressions for their implementation in numerical relativity codes. As a
proof of concept, we study the so-called ``stealth-scalarisation'' induced by
the spin of the remnant black hole after merger. As in previous studies using
alternative gauges we find that the scalarisation occurs too late after merger
to impact on the tensor waveform, unless the parameters are finely tuned.
Naively increasing the coupling to accelerate the growth of the scalar field
risks a breakdown of the effective field theory, and therefore well-posedness,
as the evolution is pushed into the strongly coupled regime. Observation of
such an effect would therefore rely on the detection of the scalar radiation
that is produced during scalarisation. This work provides a basis on which
further studies can be undertaken using codes that employ a moving-punctures
approach to managing singularities in the numerical domain. It is therefore an
important step forward in our ability to analyse modifications of general
relativity in gravitational wave observations.Comment: 15 pages, 7 figures, 4 appendices, 1 Mathematica noteboo
Neutron star-axion star collisions in the light of multimessenger astronomy
Axions are increasingly favoured as a candidate particle for the dark matter in galaxies, since they satisfy the observational requirements for cold dark matter and are theoretically well mo- tivated. Fluctuations in the axion field give rise to stable localised overdensities known as ax- ion stars, which, for the most massive, compact cases, are potential neutron star mimickers. In principle, there are no fundamental arguments against the multi-messenger observations of GW170817/GRB170817A/AT2017gfo arising from the merger of a neutron star with a neutron star mimicker, rather than from a binary neutron star. To constrain this possibility and better un- derstand the astrophysical signatures of a neutron star–axion star (NSAS) merger, we present in this work a detailed example case of a NSAS merger based on full 3D numerical relativity simula- tions, and give an overview of the many potential observables - ranging from gravitational waves, to optical and near-infrared electromagnetic signals, radio flares, fast radio bursts, gamma ray bursts, and neutrino emission. We discuss the individual channels and estimate to which distances current and future observatories might be able to detect such a NSAS merger. Such signals could con- strain the unknown axion mass and its couplings to standard baryonic matter, thus enhancing our understanding of the dark matter sector of the Universe
Testing the limits of scalar-Gauss-Bonnet gravity through nonlinear evolutions of spin-induced scalarization
Quadratic theories of gravity with second order equations of motion provide
an interesting model for testing deviations from general relativity in the
strong gravity regime. However, they can suffer from a loss of hyperbolicity,
even for initial data that is in the weak coupling regime and free from any
obvious pathology. This effect has been studied in a variety of cases including
isolated black holes and binaries. Here we explore the loss of hyperbolicity in
spin-induced scalarization of isolated Kerr black holes in a
scalar-Gauss-Bonnet theory of gravity, employing the modified CCZ4 formulation
that has recently been developed. We find that, as in previous studies,
hyperbolicity is lost when the scalar field and its gradients become large, and
identify the breakdown in our evolutions with the physical modes of the purely
gravitational sector. We vary the gauge parameters and find the results to be
independent of their value. This, along with our use of a different gauge
formulation to previous works, supports the premise that the loss of
hyperbolicity is dominated by the physical modes. Since scalar-Gauss-Bonnet
theories can be viewed as effective field theories (EFTs), we also examine the
strength of the coupling during the evolution. We find that at the moment when
hyperbolicity is lost the system is already well within the regime where the
EFT is no longer valid. This reinforces the idea that the theories should only
be applied within their regime of validity, and not treated as complete
theories in their own right.Comment: 23 pages, 10 figure, 1 tabl
Solving the initial conditions problem for modified gravity theories
Modified gravity theories such as Einstein scalar Gauss Bonnet (EsGB) contain
higher derivative terms in the spacetime curvature in their action, which
results in modifications to the Hamiltonian and momentum constraints of the
theory. In principle, such modifications may affect the principal part of the
operator in the resulting elliptic equations, and so further complicate the
already highly non-linear, coupled constraints that apply to the initial data
in numerical relativity simulations of curved spacetimes. However, since these
are effective field theories, we expect the additional curvature terms to be
small, which motivates treating them simply as an additional source in the
constraints, and iterating to find a solution to the full problem. In this work
we implement and test a modification to the CTT/CTTK methods of solving the
constraints for the case of the most general four derivative, parity invariant
scalar-tensor theory, and show that solutions can be found in both
asymptotically flat/black hole and periodic/cosmological spacetimes, even up to
couplings of order unity in the theory. Such methods will allow for numerical
investigations of a much broader class of initial data than has previously been
possible in these theories, and should be straightforward to extend to similar
models in the Horndeski class.Comment: 9 pages, 4 figures, comments welcome
GRDzhadzha: A code for evolving relativistic matter on analytic metric backgrounds
GRDzhadzha is an open-source code for relativistic simulations of matter
fields on curved spacetimes that admit an analytic description (e.g. stationary
black holes). It is based on the publicly available 3+1D numerical relativity
code GRChombo. Such a description is valid where the density of the matter is
small compared to the curvature scale of the spacetime, which is the case for
many physical scenarios - for example, dark matter environments. The approach
offers significant savings on memory and speed compared to running full
numerical relativity simulations, since the metric variables and their
derivatives are calculated analytically, and therefore are not evolved or
stored on the grid. This brief paper introduces the code and gives details of
some applications for which it has already been used.Comment: Submitted for review in the Journal of Open Source Software; Comments
welcome; The code can be found at https://github.com/GRChombo/GRDzhadzha.gi